Data processing systems are increasingly being used for applications whose operating cycle must not be interrupted. Examples of such applications are the control of industrial processes, the monitoring of space missions or the handling of banking transactions. Maintenance work or defects in such systems should only reduce their performance temporarily, not lead to a complete standstill of the job at hand.
For this purpose, such applications are typically carried out by systems with built-in redundancy where the components that are still intact take over the work of those that have failed. The necessary switching (i.e., reconfiguration) is accomplished by the systems themselves as soon as an error condition is detected.
Redundant systems are described, for example, in the article "Development of On-Board Space Computer Systems" by A. E. Cooper and W. T. Chow in the IBM Journal of Research and Development, January 1976, and in the German Offenlegungsschrift No. 2,108,836.
The expense involved in the design and manufacture of such redundant systems is substantial, as generally both the circuits (i.e., the hardware) and the control programs (i.e., the operating systems) have to be suitably adapted. This expense is often intolerable for many applications.